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There is far more water on the Moon than previously thought and it is likely widespread deep under its surface, according to a new report.

Recent Moon missions have shown frozen water in shadowed craters on the Moon's surface, and ice under the grey dust. Some scientists believe this water could have been carried there by pieces of comets hitting the surface.

"We found that the minimum water content ranged from 64 parts per billion to 5 parts per million - at least two orders of magnitude greater than previous results."

The water is not immediately accessible - it is incorporated in the rocky interior of the Moon, according to the report.

Most scientists now believe the Moon was formed when a Mars-sized object hit the Earth 4.5 billion years ago, knocking off material that compacted to form the Moon.

Magma was formed during this process and some water molecules would have been preserved as the magma cooled and crystallised.

The researchers looked at samples collected 40 years ago during Apollo Moon missions. The kinds of rocks that will be more common in the interior carry chemical evidence of hydrogen and oxygen compounds that point to water.

"The concentrations are very low and, accordingly, they have been until recently nearly impossible to detect," say Dr Bradley Jolliff of Washington University in St. Louis, who worked on the study.

"We can now finally begin to consider the implications and the origin of water in the interior of the Moon."